RTD-vs-Thermistors

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Temperature Accuracy of Thermistors and RTDsApplication Notes

Building Automation Products, Inc., 750 North Royal Avenue, Gays Mills, WI 54631 USA

Tel: +1-608-735-4800 Fax: +1-608-735-4804 E-mail:[email protected] Web:www.bapihvac.comPage 1 of 5

03/13/2008

Thermistors and RTDs are devices used tomeasure temperature in modern Heating, Ventilating, Air Conditioning andRefrigeration (HVAC/R) systems. The electrical resistance of both devices is determined by their temperature.Measuring eachdevice s resistance allows one to determine the ambient temperature of either sensor. There are tradeoffs with each device, let s see what they are.

What is an RTD, how is it definedand what is itsideal accuracy?It has been known for several hundred years that metalsincrease in resistance with an increase in temperature.Resistance Temperature Detectors (RTD) aremetal based temperature sensorsthat exploit this resistance change.RTDs can be made from many different metals to measure temperature, see the table below;

Table 1;Temperature Coefficient of Common RTD Metals

MetalRelative Conductivity

(Copper = 100% @ 20C)Temperature Coefficient of

ResistanceAnnealed Copper 100% 0.00393 / /C

Gold 65% 0.0034 / /CIron 17.7% 0.005 / /C

Nickel 12-16% 0.006 / /CPlatinum 15% 0.0039 / /C

Silver 106% 0.0038 / /C

The temperature coefficient of resistance is defined as the resistance of the RTD at 100C minus the resistance at 0Cdivided by 100. The result is then divided by the resistance at 0C. The temperature coefficient of resistance is theaverage resistance change from 0Cto 100C, the actual change for each and every degree from 0Cto 100Cis veryclose but not identical to it.

Copper has the most linear change in resistance for a given temperature change. Copper s low resistance makes itdifficult to measure small changes in temperature. Nickel has a large change in resistance with a temperature change.Nickel is not a very stable material; its resistance varies considerably from batch to batch. While nickel is much lessexpensive than platinum, the added processes needed to stabilize nickel makes nickel sensors more expensive thanplatinum.

Platinum has become the defacto standard in precision thermometry. It has a reasonably high resistance, has a goodtemperature coefficient,does not react with most contaminant gasses in airand is extremely stable from batch to batch.

In 1871 Werner von Siemens invented the Platinum Resistance Temperature Detector and presented a three terminterpolation formula. Siemens RTD rapidly fell out of favor due to the instability of the temperature reading.

Hugh Longbourne Callendar developed the first commercially successful platinum RTD in 1885. Callendar discoveredthat the insulator Siemens used embrittled the platinum causinginternal stresses that produced the temperatureinstability. Callendar changed the insulator material and annealed the RTD at temperatures above the highest desiredmeasurement temperature.

In 1886 Callendar authored a paper discussing his RTD and presented a third order equation that defined the resistanceof the RTD for the temperature range of 0 to 550C. In 1925 Milton S. Van Dusen, a researcher at the National Bureau ofStandards now NIST, extended the formula to -200C while researching test methods for refrigeration insulation.

The Callendar-Van Dusen equation has been around for 100 years, even though it is not the best fit for platinum RTD s.Callendar and Van Dusen performed their work well before the advent of modern digital computers. They could not use

much more than a third order equation since they had to solve the equation by hand. They used an equation that wasreasonably accurate and could be solved in a human lifetime.

In 1968 the International Electrotechnical Commission recognizingof the short comings of the Callendar-Van Dusenequationdefined a 20 term polynomial equation for the resistance versus temperature curve for 100 Ohm platinum RTD s(For 1,000 Ohm RTD s just multiply by ten.). In Callendar and Van Dusen s day a 20 term polynomial would have takenseveral days to solve per temperature point. The arrival of the digital computer makes solving such an equation trivial.

IEC 751 is the International Electrotechnical Commision s standard that defines the temperature versus resistance for 100, 0.00385 / /C platinum RTDs. 1,000 , 0.00385 / /C platinum RTDs are defined as ten times the IEC 751

specification.
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IEC 751 defines two classes of RTD; class A and class B. Class A RTDs operate over the temperature range of -200Cto 650C. Class B RTDs operate over the temperature range of -200Cto 850C. Class B RTDs have about twice theuncertainty of class A RTDs. See Figure 1.

The uncertainty equations for class A and class B RTDs are;

Permissible uncertainty - Class A C=(0.15 + 0.002T)

Permissible uncertainty - Class B C=(0.3 + 0.005T)

Where T = desired temperature in degrees Celsius.

0.00385 RTD Accuracy

-5

-4

-3

-2

-1

0

1

2

3

4

5

-200 0 200 400 600 800

Temperature in Degrees Celsius

ErrorinDegreesCelsius

Class A RTD Class B RTD

Figure 1;RTD uncertainty

The RTD transfer function can vary anywhere between the limitlines of Figure 1. The transfer function of an RTD is not

perfectly linear. Careful examination of the resistance versus temperature table shows a slight bow of about 0.45Cforevery 100C. Figure 2, below, shows the 1K 0.00385 RTD resistance versus temperature curve with the blue line, thered line shows an ideal straight line response.

IEC Resistance vs Temperature

0.0

500.0

1000.0

1500.0

2000.0

2500.0

3000.0

3500.0

-250 -200 -150 -100 -50 0 50 100 150 200 250 300 350 400 450 500 550 600 650 700

Temperature degrees Celsius

Resista

nceinOhms

Figure 2; RTD Transfer Function Showing RTD Resistance Bow


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What is a Thermistorhow is it defined and what is its ideal accuracy?

A thermistor is an electrical device that varies its electrical resistance with temperature(Thermistor is short for thermalresistor). The change in resistance with temperature follows the classic logarithmic curve, see figure 3.

Resistance versus Temperature BAPI 10K-2

0.00

100,000.00

200,000.00

300,000.00

400,000.00

500,000.00

600,000.00

700,000.00

800,000.00

900,000.00

1,000,000.00

-60 -45 -30 -15 0 15 30 45 60 75 90 105 120 135 150

Temperature in degrees Celsius

ResistanceinOhms

Figure 3;Temperature versus Resistance for BAPI 10K-2 Thermistor

Thermistors are made from mixtures of powdered metal oxides; recipes are closely guarded secrets of the variousthermistor manufacturers. The powdered metal oxides are thoroughly mixed and formed into the shape needed for thethermistor s manufacturing process. The formed metal oxides are heated until the metal oxides melt and turn into aceramic. Most thermistors are made from thin sheets of ceramic cut into individual sensors. The thermistors are finishedby putting leads on them and dipped into epoxy or encapsulated in glass.

Samuel Ruben invented the thermistor in 1930. Mr. Ruben worked for the Vega Manufacturing Corporation. Vega madeguitars, banjos and recording machines. Mr. Ruben was working on electronic record stylus pickupswhen he noticed thatthe pickup configuration he was working on had a rather large negative temperature coefficient.

Thermistors have come a long way in the past 80 years. According to a researcher at the National Institute of Standardsand Technology (NIST), glass encapsulated thermistors are more stable than RTDs. Thermistors, whetherglass or epoxycoated can maintain 0.2Cover large temperature intervals. Extra Precision (XP) thermistors maintain 0.1C.

By the 1960s thermistors were main stream sensors. Steinhart and Hart, two researchers at the Woods HoleOceanographic Institute, published a paper defining a temperature versus resistance formula for thermistors. TheSteinhart-Hart equation has become the industry standard equation for thermistors.

The classic Steinhart and Hart equation has the form:

1/T = A0 + A1(lnR) + A3(lnR)3

Where: T = Temperature in Kelvins (Kelvin = Celsius + 273.15)A0, A1, A3 = Constants derived from thermistor measurements

R = Thermistor's resistance in Ohms

ln = Natural Log (Log to the Napierian base 2.718281828 )

In practice three thermistor resistance measurements at three defined temperature are made. These temperatures areusually the two endpoints and the center point of the temperature range of interest. The equation hits these three pointsdirectly and has a small error over the range. BAPI can provide Steinhart-Hart coefficients for the temperature range of0Cto 70Cthat have uncertainties of 0.01Cor less.


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There are no industry or governmental standards for thermistors. There are at least 5 different temperature versusresistance curves for 10K thermistors in the HVAC/R world. All the thermistors have 10,000 Ohms of resistance at 77For 25C, but they vary greatly the further you get away from 77F. Both BAPIs 10K-2 and 10K-3 thermistors have 10,000Ohms of resistance at 77F. At 32F (0C) the 10K-2 thermistor has 32,650 Ohms of resistance and a 10K-3 29,490Ohms. If a 10K-3 thermistor is substituted for a 10K-2 you could have 6F of measurement error at 32F.

Thermistors have a very large resistance change with temperature. Differentiating between one degree and another isrelatively easy. This large resistance change limits the temperature range that can be resolved to a fraction of what anRTD can resolve.

How do the accuraciesand temperature rangesof RTDs and Thermistors compare?

Thermistors are generally more accurate than Class B RTDs over the thermistors operating temperature rangeand similato Class A RTDs.

Temperature Sensor Errors

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-4

-3

-2

-1

0

1

2

3

4

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-200 0 200 400 600 800

Temperature in Degrees Celsius

UncertaintyinDegreesCelsius

Class A RTD Class B RTDStandard Thermistor

XP Thermistor

Figure 4;Accuracy Limits and Useable Temperature Ranges for Thermistors and RTDs

Are there other application limits for RTDs and Thermistors?

The wiring used to connect the temperature sensor to the measuring device adds resistance and measurement error.

Usually 18 Gauge copperwire is used to connect sensors to their measurement devices. At 20C(43F) 18 Gauge wirehas a resistance of 6.4 Ohms for every 1000 feet of wire. At 140F (70C) 18 Gauge wire has 7.7 Ohms for every 1000

feet of wire. The table below shows how much wire may be used if you wish to keep wiring error at F or lower.Table 2;Wire lengths for F Sensor Error

Sensor Type Wire length at 20C Wire length at 70C100 Ohm RTD 15 Feet 12.5 Feet1,000 Ohm RTD 150 Feet 125 Feet10K Thermistor 98,000 feet 9,090 Feet

The wiring errors in Table 2 illustrates why temperature transmitters are used with RTDs. Reasonable wiring lengths areonly permissible with transmitters. Transmitters change the RTD resistance into a 4 to 20 mA current signal proportionalto the RTDs temperature. A temperature range must be established; a 4 mA output corresponds to the minimumtemperature and 20mA corresponds to the maximum temperature. Any intermediate temperature is just a linear


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proportion from 4 mA to 20mA. Transmitters must be within 10 feet of the RTD s location. Transmitters may be up to77,000 feet from the measurement device.

Temperature transmitters can have spans from 16.6C(30F) to 555C(1,000F) and low, 4mA temperatures, from -150C (-238F) to 482C (900F). For extra money RTDs and transmitters can be matched to have measurement errorsof 0.05C (0.1F) across the span.

So what sensor is better?

It depends.

Thermistors cost less than RTDs.

Thermistors measure temperature to the same or better accuracies than RTDs.

Thermistors do not need the extra cost of transmitters.

RTDs have a much larger temperature measurement range than thermistors.

Transmitters add at least $100 to the cost of an RTD.

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RTD-vs-Thermistors